84 THE ART OF PROJECTING. 



the sun is near the horizon, so that the porte lumiert 

 is nearly horizontal, more of these reflections will ap- 

 pear, due to multiple reflections upon the surfaces of 

 the mirror. These can be magnified a good deal in 

 the following way. Place the lens o at about its focal 



Fig. 61. 



length distant from the orifice, and then hold another 

 plane mirror r so that it will reflect the beam upon an- 

 other screen j, moving the mirror r to such a place as 

 to project the image of the orifice. It will be seen to 

 be double, and when the images overlap, the light will 

 be much brighter. Multiple reflections from the two 

 surfaces of the mirror r may be seen by holding it at a 

 small angle to the beam of parallel rays. A piece of 

 plate glass two or three inches square answers for this 

 experiment. 



That the reflected beam moves through twice the 

 angle of the incident beam, may be shown by holding 

 the mirror r in the beam without the lens o. If the 

 mirror be perpendicular to the beam, the light will be 

 reflected back through the aperture ; turning the mirror 

 slowly when it is 45 to the incident light, the beam 

 will be overhead 90 ; when it has been turned 90, and 

 is now in the plane of the beam, the reflected part will 

 have moved through 180. 



